This invention relates generally to a class of devices known as magnetic field replicators and to methods for their creation, operation and uses. Although it is not so limited, it is particularly useful for replicating complex fields with high precision, low weight and low cost, and in short real times. It may also replicate strong or very strong magnetic fields, and can produce magnets of large surface area. In addition, these replicators can also be used in much smaller devices such as for field magnets of generators.
There are many applications for which a high strength magnetic field and/or a complex, highly precise field are not only highly desirable but virtually mandatory. The conventional procedure has been to employ windings about an iron core machined to produce the shape and intensity of field desired. While this approach has been adequate in many circumstances, it suffers from some notable deficiencies. Typically, the iron or ferrous material core must be machined to a very close tolerance, often 0.001 of an inch, which makes the resulting product quite expensive. Obtaining such a precisely-machined magnet several meters in size to produce a field strength of 20,000 gauss might cost on the order of a hundred thousand dollars or much more, and the final product might well weigh from one ton to tens of tons. Higher strength magnets of conventional design are obtainable only at still higher costs and weights, perhaps exponentially higher. Additionally, since iron magnets wound with conventional wire dissipate a great deal of power, sometimes hundreds of kilo-watts, operating such magnets can be both quite costly and difficult to schedule with power companies.
Since iron saturates in the region of 20,000 to 30,000 gauss, higher fields are presently created by windings of superconducting wire which, due to the high fields, are subject to tremendous forces. If such wire moves or flexes by just a very small amount, the superconducting property is often lost or, in the terms of the art, a “quench” is said to have occurred. Creating high strength superconducting magnets requires an exceedingly great precision of support and windings, at a corresponding cost. For example, a solenoidal, superconducting wire-wound magnet, two meters long and one meter in diameter, might cost several million dollars.
Attempts have been made, in instances where a number of identical field sources were desired, to manufacture only one such expensive conventional magnet and to duplicate the field by placing a magnetizable material in close proximity to the high strength field. With the discovery of superconducting material—i.e., material capable of conducting an electric current with no resistance and no losses—it was thought logical to try to impress the desired field pattern in a magnet of such material. However, another characteristic of such materials is the tendency to expel all internal magnetic fields when the critical temperature, Tc—the temperature below which superconductivity occurs—is achieved. This characteristic is now known as the Meisner Effect. Conflicting characteristics of the superconducting materials therefore existed—i.e., a material thought to be ideal in many respects was at hand, but the superconductivity material would expel the impressed magnetic field upon achieving the superconducting state.
In the mid-1970's, Mario Rabinowitz and colleagues at the Stanford Linear Accelerator Center, using materials which exhibited superconductivity only when within a few degrees of absolute zero, discovered that by causing imperfections in the material, for example by work hardening, the magnet would not expel all magnetic lines of force from within the material when, while subject to a magnetic field, its temperature was taken below the critical temperature. This discovery was dubbed the “Incomplete Meisner Effect”, and the retention of a very strong magnetic field by superconducting materials is now known as the “Very Incomplete Meisner Effect”, or VIME.
Superconducting materials for which the Meisner Effect is literally true—i.e., materials which completely expel all of their internal magnetic fields when the magnetic field strength is less than the critical field strength, HC—are known as Type I superconductors. Superconducting materials for which the Meisner Effect is not literally true—i.e., those which demonstrate either the Incomplete Meisner Effect or the Very Incomplete Meisner Effect—are known as Type II superconductors. Additionally, in Type I SC's, the magnetic field is totally penetrating—i.e., completely contained within the body of the SC—when the strength of the magnetic field is greater than the critical field strength, HC, and the property of superconductivity is lost, whereas in a Type II SC, the magnetic flux lines penetrate the body when the field strength is greater than a first critical level HCl, but the superconductivity property is not lost until the field strength exceeds a second critical level, HC2. This “window”, for the Incomplete or Very Incomplete Meissner effect, allows the theoretical possibility for retention of strong magnetic fields in bulk SC materials, which is fundamentally different from creation of such fields by currents through wires or ribbons.
Further, all superconducting materials must be maintained at temperatures below their respective critical temperatures, Tc, in order to maintain their superconductivity; the respective temperatures at which such materials lose their superconductivity properties forms the basis for a further classification of such materials. Superconductors which are able to maintain their superconductivity property only within a few degrees of absolute zero are known as low critical temperature or low Tc superconductors, while superconductors which are able to maintain that property at much higher temperatures are known as high Tc superconductors. In a practical sense, materials which become superconducting below the temperature of liquid nitrogen require liquid helium as a coolant. Materials which become superconducting above the temperature of liquid nitrogen can use liquid nitrogen as a coolant. Since liquid nitrogen is much cheaper and easier to handle than liquid helium, it is convenient, from a practical standpoint, to take the dividing line between low Tc and high Tc superconductivity as the temperature of liquid nitrogen even though this temperature may not represent the actual demarcation line between these two classes of materials. In a historical sense, the high Tc superconductors were the first to exhibit superconductivity above temperatures of about 30 Kelvin.
Rabinowitz' work, so far as is known to date, is limited to the class of superconductors known as metallic superconductors, which to date are further limited to temperatures within a very few degrees of absolute zero (30 K or less). This limitation requires the use of liquid helium as the coolant, which not only greatly increases the cost but which, due to extreme difficulties of handling, greatly limits the practical applications of such devices. According to “Dependence Of Maximum Trappable Field On Superconducting Nb3Sn Cylinder Wall Thickness”, M. Rabinowitz et al., Applied Physics Letters 30, 607, 1977, Rabinowitz's results were achieved through the use of multiple layers of Nb3Sn foils formed in the shape of a cylinder by helically wrapping the ribbons of superconducting foil around a mandrel. While satisfactory for laboratory experiments, the practical limitations of such delicately-constructed materials are obvious.
U.S. Pat. No. 4,176,291, “Stored Field Superconducting Electrical Machine And Method”, to Rabinowitz, discloses the employment of a metallic superconductor known as “A-15, beta-tungsten structure” superconductor formed in the shape of cylinders. These cylinders are formed in concentric layers of superconducting materials and thermally and electrically conductive materials in order to insulate the magnetic field replicator from thermal and electromagnetic forces and from heat build up which might otherwise cause the extremely low Tc of the Rabinowitz device to be exceeded, with resultant loss of superconductivity and the impressed magnetic field. Rabinowitz discloses the use of the “warm process” method to impress the desired field in his specialized cylinder, and states that with that process he has been able to store a magnetic field in the superconductor of up to about one-half Hc2.
U.S. Pat. No. 4,190,817, “Persistent Current Superconducting Method And Apparatus”, to Rabinowitz, discloses, for low Tc, metallic superconductors, various means for varying the field, for creating complex and/or spatially large fields, and for miniaturizing fields, as well as means for increasing the fidelity, magnitude, and stability of the magnetic field stored in such low Tc superconductors.
U.S. Pat. No. 4,096,403, “Superconducting Hybrid Magnetic Flux Pump”, also to Rabinowitz, discloses various arrangements for magnetic flux pumps. None of the Rabinowitz references discloses high Tc, bulk superconductors.
With the relatively recent discovery of the new class of superconducting materials known, relatively, as ‘high temperature’ superconductors, a number of researchers have attempted to use such materials to replicate high intensity magnetic fields. However, it has been found that this may not be accomplished simply by extending Rabinowitz' work to such materials; rather, this new class of superconducting materials has its own, unique problems which must be overcome before such materials may successfully be used as magnetic field replicators.
One such problem not found with low Tc SC's, and a paramount problem for high Tc SC's, is the tendency for the retained or “trapped” field strength, BT, of such materials to undergo time decay. Representative of this relationship is the graph of FIG. 15 of retained field strength, BT, as a function of time displayed logarithmically:
The field strength, which decays with time logarithmically, does so initially at a precipitous rate, and then abruptly changes to a more moderate rate of decline. The decay rate following this inflection point is referred to as “Creep”, or “flux creep”, analogous to the dislocation line movement in crystalline materials. With low Tc SC's, some of the field strength is lost—i.e., some magnetic flux lines escape—within very short time periods (on the order of 10 seconds), but thereafter the field appears to stabilize and, within the time limits of the Rabinowitz experimenters, the loss appears to be virtually zero. In the aggregate, with low Tc materials, creep is minor, and the trapped field may persist for millenia. In the newer high Tc materials, the creep can reduce the trapped field by several percent in one week and is referred to as “Giant Creep”. While some applications are possible in high Tc materials in spite of the Giant Creep phenomena, many more applications would be possible if the phenomena were eliminated, significantly reduced, or otherwise overcome or controlled.
Still another difficulty confronting the high temperature superconductivity researcher is that the superconductivity property is not isotropic, i.e., is not uniform in all directions. Typically, the SC property is manifested in only two directions, i.e., a plane, and is smaller, or non-existent in a direction perpendicular to this plane. It was long thought that the SC phenomenon was confined to a plane at the surface of a superconducting conductor, and it was therefore thought that this limited available volume for trapping would impose significant limitations on the strengths of the fields which could be trapped. Simultaneously, since it was also thought that the fields could be trapped only in the direction perpendicular to the SC plane, serious reservations existed in most quarters of about the prospects of ever developing strong, practical, high Tc SC magnetic replicators.